1. Field of the Invention
The present invention is directed to solid pyrotechnic compositions and methods for making the same, including novel black powder substitute and boron/potassium nitrate substitute compositions. More particularly, the present invention is directed to solid pyrotechnic compositions, and methods for making the same, which compositions have increased moisture resistance (measured in terms of humidity uptake) in comparison to black powder and boron/potassium nitrate compositions.
2. State of the Art
Black powder and boron/potassium nitrate (B/KNO3) are conventional igniter formulations with broad current usage in a wide variety of military applications. For instance, black powder and B/KNO3 are important components in ignition, propulsion and expulsion trains of many modern military weapons systems. Additionally, black powder is commonly used in commercial applications such as muzzle loading rifles, fireworks and model rocket propulsion systems, whereas B/KNO3 is commonly used in the ignition trains of automotive restraint systems.
Black powder is typically comprised of between 72 and 75 weight percent potassium nitrate, between 15 and 18 weight percent charcoal and 10 weight percent sulfur. Variations in this basic black powder formulation are known to those of ordinary skill in the art. The optimum formulation for black powder is generally accepted to consist of 75 weight percent potassium nitrate, 15 weight percent charcoal and 10 weight percent sulfur. Black powder of this optimum formulation has a predicted flame temperature of about 1950K at 1000 psi.
Boron/potassium nitrate, on the other hand, in its optimum formulation, is generally accepted to consist of 75 weight percent potassium nitrate and 25 weight percent boron. Compared to black powder, B/KNO3 has a significantly higher flame temperature of about 3034K at 1000 psi.
As the flame temperatures of black powder and B/KNO3 differ significantly, the applications in which these igniter compositions are used differ somewhat. Black powder is by far the less expensive of the two compositions, has a cooler flame temperature and produces less slag than B/KNO3. For these reasons, black powder is often chosen over B/KNO3 in the ignition trains of multiple-use hardware, including guns of various sizes and applications. Boron/potassium nitrate is typically utilized in applications where a higher flame temperature is critical for rapid and reproducible ignition. For example, common applications for B/KNO3 include ignition trains for rockets, decoy flares and gas generators of automotive secondary safety restraints or “air bag” devices. Boron/potassium nitrate is not, however, used as often in multiuse hardware due to the expense of B/KNO3 and the high B/KNO3 flame temperature, which may cause premature erosion of reusable hardware.
Processes are known to those of ordinary skill in the art for making black powder. For example, charcoal and sulfur may be ball milled together into an intimate mixture. Ball milling also serves to reduce the particle size of the charcoal and sulfur. Potassium nitrate is dried and likewise processed through a rod mill to reduce the average particle size to about 50 microns. The milled charcoal, sulfur and potassium nitrate are then compounded, milled, and optionally coated with graphite, in accordance with well-known methods.
Despite its widespread use, certain characteristics of black powder make it highly desirable to replace it in some (or all) of its common applications. For example, safety is a major concern during black powder production. Further, the combustion of black powder produces a plethora of effluents. It has been calculated that the black powder combustion generates significant amounts of carbon monoxide, sulfur dioxide and hydrogen sulfide. Potassium sulfide has been predicted to constitute over 20 percent of the combustion products. At flame temperature, potassium sulfide is produced in the liquid state and is likely to undergo after-burning with atmospheric oxygen to produce copious amounts of sulfur dioxide. The carbon monoxide and hydrogen sulfide are also susceptible to after-burning, yielding carbon dioxide and sulfur dioxide, respectively.
Sulfur dioxide is extremely destructive to tissue of the mucous membranes and upper respiratory tract, eyes and skin Inhalation may result in spasm, inflammation and edema of the larynx and bronchi, chemical pneumonitis and pulmonary edema. Thus, exposure to sulfur dioxide can lead to a series of health problems and, in the case of extended exposure, death.
Another concerning characteristic of black powder is its reproducibility. The charcoal constituent of black powder imparts a degree of unpredictability to the performance of the igniter composition. Charcoal is produced by carbonization of wood. As described in U.S. Pat. No. 5,320,691 to Weber, the chemical and physical properties of wood vary greatly, depending upon the particular properties of the tree species, soil composition and environmental conditions from which the wood is taken. Due to the inherent variability of wood and fluctuations in the carbonization process, the properties of charcoal tend to vary from batch to batch. These variations can effect the consistency of black powder performance.
Yet another problem associated with black powder is its hygroscopicity. Black powder absorbs about 1.5 weight percent moisture under 75 percent relative humidity at a temperature of 21.1° C. (70° F.) over a period of 24 hours. If black powder picks up sufficient moisture, there is a possibility that the black powder will not burn as fast. Hence, an igniter or other device comprising the black powder might not perform up to specification in a high relative humidity. Also, concerns have been expressed that water will cause the potassium nitrate to migrate out of the black powder pellet and cause corrosion of metallic parts of the device.
In light of the above-described concerns, there is a continuing need in the pyrotechnic industry for black powder substitutes which are safer to produce, more predictable and less hygroscopic than black powder. One black powder substitute composition is described in U.S. Pat. No. 5,320,691 to Weber. This composition is a dispersion of phenolphthalein, potassium nitrate and sulfur in a binding phase of phenolphthalein salt. Phenolphthalein is the reaction product of a phenolic compound and phthalic anhydride. The cations of the phenolphthalein salt are selected from the group consisting of sodium, potassium, lithium and ammonium. Phenolphthalein salt (optionally in combination with organic phenolphthalein) is used because of the ballistic enhancement that the phenolphthalein salt imparts in comparison to organic phenolphthalein. Although the substitution of phenolphthalein salt for charcoal obviates the predictability problems raised by the charcoal of the conventional black powder composition, sulfur remains as a requisite ingredient of this substitute composition. Thus, the black powder substitute of U.S. Pat. No. 5,320,691 does not address the above-mentioned problems associated with sulfur and sulfur dioxide production. Also, phenolphthalein salts are hygroscopic and do not overcome concerns regarding moisture uptake.
Another black powder substitute is described in United States Patent & Trademark Office Disclosure Document H72 to Wise, et al. The solid pyrotechnic composition described therein contains 75 weight percent potassium nitrate, 10 weight percent elemental sulfur and 15 weight percent crystalline compound. The crystalline compound may be fluorescein, phenolphthalein, 1,5-naphthalenediol, anthraflavic acid, terephthalic acid and alkali metal salts thereof. As in the case of other known black powder substitute compositions, the substitute composition described in Disclosure Document H72 relies on elemental sulfur for minimizing the ignition delay of the igniter and, thus, does not address concerns regarding sulfur and sulfur dioxide production.
Accordingly, the inventors of the present invention have recognized that alternative substitute compositions for black powder and boron/potassium nitrate, and methods for making such compositions, would be advantageous.